Microscope Lens Using Conversion of Rayleigh to Compressional Waves

A planar a coustic microscope lens is demonstrated which produces an almost aberrationfree focal spot in a liquid mediuc. A circular interdigital transducer generates converging acoustic surface waves on the interface between the piezoelectric solid and the liquid and these Rayleigh waves convert to a hollow cone of compressional waves converging to a focus on the axis in the liquid. The measured amplitude distribution in the r egion of the focus agrees with calculations and the p rototype l ens has been used at 20 MHz with a mechanical sweep to produce acoustic images. The simplicity of the planar photolithographi c fabrication i nvites modified electrode shapes and excitation to change tho focal p attern.